1. Field of the Invention
The present invention relates to an optical signal quality monitor for monitoring, for example, a signal-to-noise ratio, polarization mode dispersion or the like of an optical signal in an optical transmission system.
2. Description of the Background Art
When investigating the quality or deterioration factor of a transmitted signal, an estimate of the quality or factor may require the waveform of the signal to be monitored.
The waveform of an optical signal may be monitored by using devices such as a sampling oscilloscope. Generally, the sampling oscilloscope is however a complicated and expensive device. As opposed to such a device, there is also a proposal for monitoring the quality of an optical signal without using the sampling oscilloscope.
Japanese patent laid-open publication No. 2005-151597 discloses an optical signal quality monitor that samples an optical signal to be measured having a predetermined bit rate by using an asynchronous timing clock, and uses the resulting histogram to determine the signal-to-noise ratio coefficient Q. This monitor scheme can obtain information on the signal waveform in a short time and configure a monitoring device relatively easily.
U. K. Lize et al., “Simultaneous Monitoring of Chromatic Dispersion and PMD for OOK and DPSK Using Partial-Bit-Delay-Assisted Clock Tone Detection” Proc. 31st European Conf. on Opt. Commun. (ECOC2003) Mo4. 4 Jul. 2006 proposes that it is possible to detect a frequency component having high intensity from an optical signal and use the frequency component to detect polarization mode dispersion (PMD) and chromatic dispersion.
However, the monitoring method of the optical waveform taught by the above-mentioned Japanese '597 patent publication is subject to a difficulty that the higher bit rate of the optical signal to be observed the fewer sampling points, thus rendering it difficult to analyze the signal.
The above-mentioned U. K. Lize, et al., discloses the measuring method of detecting the frequency component corresponding to the bit rate of an optical signal, thus also confronting the difficulty in analyzing the higher-bit rate signal.
Recently, an optical transmission system for transmitting and receiving an RZ (Return-to-Zero) optical signal of a 160 Gbps bit rate has been researched and developed. However, the methods taught by the above-mentioned Japanese '597 patent publication and U. K. Lize, et al., may not be satisfactory to such a high-bit rate optical signal.